Method for operating an electrical network

ABSTRACT

Method for operating an electrical network including a first subnetwork and a second subnetwork which are connected to one another via a transformer and are DC-isolated from one another by the latter. A primary side of the transformer with a first number of turns is assigned to the first subnetwork and a secondary side of the transformer with a second number of turns is assigned to the second subnetwork. The first subnetwork has a multi-level converter having a plurality of individual modules, and each individual module has an electrical energy store. The multi-level converter provides at least one first incoming electrical AC voltage which is modulated with at least one second incoming electrical AC voltage. A resulting electrical voltage is made available to the transformer and is transformed by the transformer to an outgoing electrical voltage which is made available to the second subnetwork.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to German Patent Application No. DE 10 2016 105 542.5, filed Mar. 24, 2016, which is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The invention relates to a method for operating an electrical network, to a multi-level converter and to an energy supply system.

BACKGROUND OF THE INVENTION

An electrical network may have a plurality of energy sources which can be used to provide a plurality of consumers, which are connected to the electrical network, with electrical energy. In this case, it is also possible for the electrical network to be subdivided into a plurality of subnetworks which are each assigned different energy sources and consumers. The different subnetworks may have different voltages which are to be used to operate the subnetworks, these different voltages having different amplitudes and/or different maximum values. Two subnetworks which are connected to one another in this case and have different voltages are connected to one another via a voltage converter, for example a DC-DC converter or an AC-AC converter.

U.S. Pat. No. 5,093,583 A, which is incorporated by reference herein, discloses an electrical system for a motor vehicle comprising a low-voltage network and a high-voltage network. In this case, a generator produces a low voltage which supplies the low-voltage network and a transformer of the motor vehicle. This transformer is designed to convert the low voltage into a high voltage which is to be used to also operate consumers of the high-voltage network in parallel with consumers of the low-voltage network.

A method for supplying an electric motor with an alternating current is described in US 2010 0 140 003 A1, which is incorporated by reference herein. In this case, depending on the requirement of the electric motor, the latter is provided with an electrical voltage using at least pulse width modulation, a choice being made between a plurality of types, for example three types, of pulse width modulation to be used in each case.

US 2013 0 106 365 A1, which is incorporated by reference herein, discloses the practice of charging an energy store of an electrical motor vehicle via an external energy source. In this case, it is possible to charge the energy store of the motor vehicle with the external energy source in a DC-isolated manner or directly.

A fuel cell system which can be used to supply electrical loads with electrical energy is described in US 2014 0 152 089 A1, which is incorporated by reference herein. In this case, an inverter is arranged between a fuel cell and an electrical load in each case and is designed to produce a multiphase high voltage required by the respective load, interfering noises being avoided by selecting a difference in phases of the high voltages.

US 2014 0 225 432 A1, which is incorporated by reference herein, discloses a current transformer which comprises three coils and is designed to exchange electrical energy between different voltage sources and voltage networks of an electrical motor vehicle.

SUMMARY OF THE INVENTION

Against this background, described herein is a method and a device which can be used to produce voltages with different maximum values, a first consumer to be provided with a first voltage having a first value not being disrupted by a second voltage having a second value which is to be made available to a second consumer.

The method according to the invention is intended to operate an electrical network comprising a first subnetwork and a second subnetwork which are connected to one another via a transformer and are DC-isolated from one another by the latter. In this case, a primary side of the transformer with a first number of turns is assigned to the first subnetwork and a secondary side of the transformer with a second number of turns is assigned to the second subnetwork. The first subnetwork has a multi-level converter having a plurality of individual modules, each individual module having an electrical energy store. The multi-level converter provides and/or produces at least one first or primary incoming electrical AC voltage as an input voltage which is modulated with at least one second or secondary incoming electrical AC voltage as a further input voltage. An electrical voltage resulting from such modulation is made available to the transformer and is transformed by the latter to an outgoing electrical voltage as an output voltage which is made available to the second subnetwork.

The first number of turns of the primary side of the transformer is usually greater than the second number of turns of the secondary side. A maximum amplitude of the resulting AC voltage of the first subnetwork and therefore of a high-voltage network is therefore greater than an amplitude of the outgoing AC voltage for the second on-board subnetwork which is accordingly in the form of a low-voltage network and/or can be referred to as a low-voltage network.

In the method, provision is made for the at least one first incoming AC voltage to have an amplitude with a first value and a frequency with a first value, and for the at least one second incoming AC voltage to have an amplitude with a second value and a frequency with a second value. In this case, the first value of the amplitude of the at least one first AC voltage is usually set to be greater than the second value of the amplitude of the second AC voltage. The first value of the frequency of the at least one first AC voltage is usually set to be less than the second value of the frequency of the at least one second AC voltage. Alternatively, it is possible for the first value of the amplitude of the at least one first incoming AC voltage to be less than the second value of the second incoming AC voltage. In addition, it is also possible for the value of the frequency of the at least one first incoming AC voltage to be greater than the value of the frequency of the second incoming AC voltage.

In one configuration, the at least one first incoming AC voltage is modulated with the at least one second incoming AC voltage at a reference point of the multi-level converter. In this case, a neutral point of the multi-level converter, for example, is selected as the reference point.

The at least one second incoming AC voltage is usually modulated onto the at least one first incoming AC voltage by implementing amplitude modulation and is therefore added to said first incoming AC voltage, the resulting voltage being provided as a sum of the incoming AC voltages.

It is also possible for the multi-level converter to provide a plurality of, for example three, incoming first or primary AC voltages or phases which are phase-shifted with respect to one another and are modulated with the at least one second or secondary incoming AC voltage. In this case, provision is made for each of the first incoming AC voltages to be modulated with a second incoming AC voltage, the phase of which is to be respectively set individually. Alternatively, all first incoming AC voltages are modulated with the same second incoming AC voltage.

In one configuration, the at least one first incoming AC voltage or phase is modulated with the at least one second incoming AC voltage and is therefore excited. The modulation carried out in this case is modulated relative to the reference point of the multi-level converter, for example between a connection of the multi-level converter and the reference point, a respective connection being provided for the at least one first incoming AC voltage or phase. If the multi-level converter is used to produce a plurality of first incoming AC voltages and therefore phases, they are compensated for with the at least one second incoming AC voltage to be used for the modulation.

The multi-level converter comprising a plurality of individual modules having energy stores is likewise in the form of an energy store or energy source and/or can be referred to as an energy store or energy source and is used to provide consumers of the subnetworks with AC voltages having different frequencies. In this case, consumers of the first subnetwork are provided with AC voltages having amplitudes and frequencies respectively adapted according to requirements.

In addition, consumers of the second subnetwork are provided with the outgoing AC voltage via the transformer on the basis of the resulting AC voltage, the frequency and amplitude of the outgoing AC voltage being dependent on the frequency and amplitude of the resulting AC voltage and on a ratio of both numbers of turns of the transformer.

The multi-level converter also has a plurality of distributed individual modules, an energy store of a respective individual module providing a DC voltage or an AC voltage, in which case, if a respective energy store provides a DC voltage, this DC voltage is converted into an AC voltage by the multi-level converter.

The multi-level converter or multi-stage converter according to the invention is to be arranged in an electrical network comprising a first subnetwork and a second subnetwork which are to be connected to one another via a transformer and are to be DC-isolated from one another by the latter, in which case a primary side of the transformer with a first number of turns is to assigned to the first subnetwork and a secondary side of the transformer with a second number of turns is to be assigned to the second subnetwork. The multi-level converter is to be arranged in the first subnetwork and has a plurality of individual modules, each individual module having an electrical energy store. The multi-level converter is designed to provide and/or produce at least one first incoming electrical AC voltage as an input voltage and to modulate it with at least one second incoming electrical AC voltage as an input voltage, in which case a resulting electrical voltage is to be made available to the transformer by the multi-level converter, is to be transformed by the transformer to an outgoing electrical voltage as an output voltage and is to be made available to the second subnetwork.

The multi-level converter is assigned a monitoring unit which is designed to set values of at least one physical parameter, for example an amplitude and/or a frequency, of the at least one incoming AC voltage. Depending on the definition, this monitoring unit is in the form of a component of the multi-level converter and/or can be referred to as a component of the multi-level converter.

Furthermore, at least two individual modules of the multi-level converter, generally all individual modules, have the same design.

The multi-level converter is designed to produce or provide the at least one first incoming AC voltage from an individual voltage from an energy source or an energy store of at least one individual module, a plurality of incoming first AC voltages being superimposed on one another and/or being temporally phase-shifted with respect to one another.

The multi-level converter is also designed to connect at least two individual modules in series and/or in parallel with one another and to provide the at least one first incoming AC voltage from a combination of individual voltages of the at least two individual modules to be combined with one another. In this case, individual modules are switched on or off depending on requirements.

The multi-level converter has a plurality of, for example three, sections, each section having a combination of a plurality of individual modules which are connected to one another and usually have the same design, in which case each section is respectively to be used to produce a first incoming AC voltage and therefore phase. The value of the amplitude of the respective first incoming AC voltage is set on the basis of which individual module of a respective section is switched on or off and how a plurality of switched-on individual modules of the section are connected in series and/or in parallel with one another.

The multi-level converter is assigned at least one additional energy source or at least one additional energy store which is designed to provide the at least one second incoming AC voltage.

The energy stores of the individual modules are generally in the form of DC voltage sources. The multi-level converter has at least one converter which is designed to convert an individual voltage in the form of a DC voltage from an energy store of at least one individual module into an AC voltage and to provide the at least one first incoming AC voltage therefrom.

The at least one second incoming high-frequency AC voltage is usually modulated onto the at least one first incoming low-frequency AC voltage.

The primary side of the transformer is also excited by the multi-level converter, that is to say by the resulting voltage provided by the multi-level converter.

The transformer has a high-pass characteristic, only portions of the resulting voltage which are at least as great as a cut-off frequency being taken into account by the transformer and being transformed to the outgoing voltage.

The energy supply system according to the invention comprises an electrical network comprising a first subnetwork and a second subnetwork which are connected to one another via a transformer and are DC-isolated from one another by the latter, a primary side of the transformer having a first number of turns and being assigned to the first subnetwork and a secondary side of the transformer having a second number of turns and being assigned to the second subnetwork. The first subnetwork comprises a multi-level converter having a plurality of individual modules, each individual module having an electrical energy store. The multi-level converter is designed to provide and/or produce at least one first incoming electrical AC voltage or input voltage and to modulate it with at least one second incoming electrical AC voltage or input voltage. A resulting electrical voltage is to be made available to the transformer. The transformer is designed to transform the resulting electrical voltage to an outgoing electrical voltage or output voltage and to make it available to the second subnetwork.

In one configuration, the first number of turns of a coil of the primary side of the transformer is greater than the second number of turns of a coil of the secondary side of the transformer. Alternatively, it is conceivable for the first number of turns of the coil of the primary side to be less than the second number of turns of the coil of the secondary side.

The energy supply system is to be arranged in a motor vehicle, for example.

Furthermore, an electrical machine having a plurality of phases is to be assigned to the first subnetwork as consumer, the multi-level converter being designed to respectively provide each phase with a first incoming AC voltage.

In one configuration, the presented multi-level converter according to the invention is in the form of a component of the presented energy supply system according to the invention, in which case the multi-level converter and/or the energy supply system is/are to be used to supply consumers of the network, that is to say at least one consumer of the first subnetwork which is usually in the form of an electrical machine and at least one consumer of the second subnetwork, with electrical energy. In this case, one configuration provides for such an electrical machine to be operated as an electrical motor which is used to convert electrical energy into mechanical energy. Alternatively or additionally, it is also possible for this electrical machine to be operated as an electrical generator depending on the requirements.

If the energy supply system and the network are intended for a motor vehicle, the network is also in the form of an on-board network of the motor vehicle and/or can be referred to as an on-board network of the motor vehicle. Accordingly, the two subnetworks are in the form of on-board subnetworks of the motor vehicle and/or can be referred to as on-board subnetworks of the motor vehicle which are to be operated with voltages, the amplitudes or maximum values of which are different. In this case, provision is also made for the electrical machine, as the consumer of the first subnetwork, the voltage of which has an amplitude with a large value if it is operated as an electrical motor, to be designed to drive or move the motor vehicle. If the electrical machine is alternatively operated as an electrical generator, it is to be used to convert mechanical energy of the motor vehicle, for example in a recuperation mode, into electrical energy, in which case electrical energy provided in the process is to be stored in an energy store of the electrical network. A consumer of the second subnetwork, the voltage of which has an amplitude with a low value, is designed to carry out a monitoring function of the motor vehicle, for example.

The presented method according to the invention is to be carried out using the multi-level converter and/or the energy supply system, in which case the method is to be monitored and therefore controlled and/or regulated using the multi-level converter and/or the energy supply system.

In one configuration, at least a primary side and therefore a primary coil or winding of the transformer is excited by the multi-level converter which is also in the form of a high-voltage multi-level converter and/or can be referred to as a high-voltage multi-level converter, for example, if the first subnetwork is to be operated with a higher voltage than the at least one second subnetwork. The value of the frequency of the at least one first incoming AC voltage, which is provided by the multi-level converter and is to be used to supply the consumer of the first subnetwork, is generally comparatively low and is at most two kilohertz. In contrast, the frequency of the at least one second incoming AC voltage, which is used to modulate the at least one first incoming AC voltage, is greater than the value of the frequency of the at least one first incoming AC voltage.

The multi-level converter is in the form of a modular multi-level converter (MMC) or MMSPC, for example. A multi-level converter in the form of an MMSPC is described in the document “Modular Multilevel Converter with Series and Parallel Module Connectivity: Topology and Control.” (IEEE Transaction on Power Electronics) by S. M. Goetz, A. V. Peterchev and T. Weyh.

The at least one first incoming AC voltage to be produced generally has a high dynamic response. The value of the amplitude of the at least one first incoming AC voltage is usually greater than the value of the amplitude of the at least one second incoming AC voltage by a plurality of orders of magnitude. So-called frequency multiplexing of the first incoming AC voltages is possible by combining a plurality of first incoming AC voltages which are superimposed by the multi-level converter, the first incoming AC voltages combined with one another in this manner being designed to supply the consumer of the first subnetwork starting from the multi-level converter.

A high-pass characteristic of the transformer can be set by selecting a value of an inductance of at least one of the two coils or of the transformer, the inductance of the respective coil being dependent on its number of turns.

The excitation of the transformer is set by the value of the frequency and/or of the amplitude of the resulting voltage provided by the multi-level converter from the incoming AC voltages. In this case, at least the amplitude and/or frequency of the first incoming AC voltages is/are adapted to requirements of the consumer of the first subnetwork. Amplitude modulation is carried out as part of the modulation to be carried out by the multi-level converter. This modulation to be carried out can be carried out only for a first incoming AC voltage and therefore a first phase or for a plurality of first incoming AC voltages and therefore for a plurality of phases, the incoming first AC voltages being mutually compensated for in the last-mentioned case.

In one configuration, the secondary side of the transformer is connected to at least one rectifier and therefore possibly to a topology comprising a plurality of rectifiers, at least one consumer of the second subnetwork in turn being connected to the at least one rectifier, the outgoing AC voltage provided by the transformer being converted into a DC voltage by the at least one rectifier. The at least one rectifier is usually active or passive and generally has at least one AC voltage or DC regulating stage which is in the form of a back stage, boost stage or back/boost stage for voltages of 110 V or 240 V, for example. The topology formed from the at least one rectifier has at least a single-pulse or multi-pulse design, for example a single-pulse to twelve-pulse design. A semiconductor module in the form of a field effect transistor (FET), for example, is to be used to actively regulate the at least one rectifier. At least one diode, for example, is to be used for passive regulation.

In a first possible embodiment of the method, of the multi-level converter and/or of the energy supply system, provision is made for the first subnetwork to be in the form of a high-voltage supply network and for the second subnetwork to be in the form of a low-voltage supply network. In this case, in one configuration, the second subnetwork has at least one separate energy store, for example a capacitor and/or a battery. In this case, an average power requirement of the first subnetwork is higher than the average power requirement of the second subnetwork by a multiple, for example a factor of five. If the energy supply system, and therefore the electrical network, is used for a motor vehicle, the average power requirement of the second subnetwork with a maximum voltage of 12 V, for example, is 1 to 3 kW. In contrast, depending on the configuration of the motor vehicle to be driven, the power requirement of the first subnetwork for driving the motor vehicle is 20 kW to 400 kW, for example.

During the modulation to be carried out using the multi-level converter, additional power is fed into the first subnetwork via the at least one second incoming AC voltage and is included in this case in dynamic power regulation of the first subnetwork. So that an instantaneous power of the first subnetwork, which results from an actual desired current or desired voltage profile on the basis of the at least one first incoming AC voltage, follows an instantaneous regulation target, said power corresponds, only on temporal average, to the power requirement of the second subnetwork and therefore its consumers and/or energy stores. These two conditions from an instantaneous power requirement of the first subnetwork and an average power of the second subnetwork determine two degrees of freedom from a degree of the modulation to be carried out and a degree of the power of a desired voltage or desired current profile.

In the second embodiment, the modulation for exciting the transformer in the first subnetwork is not visible to the at least one consumer of the first subnetwork, for example at least one electrical machine for driving the motor vehicle. Electrostatic loading of the consumers of the first subnetwork is reduced by eliminating high-frequency components of the resulting AC voltage in the first subnetwork. This concerns, for example, insulation of the at least one consumer which is in the form of an electrical machine, which defines its service life, inter alia. In one configuration, by using the degrees of freedom of the multi-level converter as physical circuit, the modulation is carried out relative to its reference point. In this case, this reference point corresponds, for example, to a neutral point of the usually multi-phase multi-level converter. In this case, the modulation is carried out between a connection of one phase of the consumer, for example of the multi-phase electrical machine, and the reference point. If the consumer has at least one neutral point, that is to say one neutral point or a plurality of neutral points, which is the case for a three-phase star winding of the electrical machine for example, it should not be connected to the reference point. Instead, the modulation is carried out in a parallel manner at other connections for phases of the consumer, the modulation being present only between the connections of the phases and therefore the reference point, but not between connections of the consumer. In this case, the reference point is used as the connection for the transformer.

The presented multi-level converter is to be used to provide a resulting voltage which has low distortion, thus avoiding interference from other electrical devices. Inside the first subnetwork, electrical energy provided by the multi-level converter is used by the electrical machine to drive the motor vehicle. In this case, it is possible to operate the electrical machine in a voltage-controlled manner starting from the multi-level converter.

The multi-level converter is, for example, in the form of a neutral-point-clamped (NPC) converter which has a neutral conductor at a neutral point, in the form of a flying capacitor, in the form of a modular multi-level converter or in the form of an MMSPC, which is to be used to produce a plurality of voltages, for example AC or three-phase voltages, for at least one electrical machine for driving a motor vehicle. Such a voltage provided for supply has a value in the high-voltage range of greater than 60 volts, usually greater than 200 volts, and is generally fed from a plurality of energy stores, for example high-voltage stores. At least one output of the multi-level converter is DC-isolated from the at least one high-voltage store. If the multi-level converter has a plurality of outputs, they are likewise DC-isolated from one another.

The electrical machine is to be supplied with energy via the first subnetwork which has the multi-level converter, the first subnetwork being in the form of a high-voltage system and/or being able to be referred to as a high-voltage system. In contrast, the second subnetwork is in the form of a low-voltage system and/or can be referred to as a low-voltage system, via which further consumers, for example lighting devices, auxiliary units, monitoring or control modules or communication devices of the motor vehicle, are to be supplied with electrical energy. The second subnetwork has a maximum voltage of 12 V, 24 V or 48 V, for example. In contrast, the first subnetwork has substantially higher voltages of 110 V or 240 V, for example.

All subnetworks are DC-isolated from one another via the transformer, with the result that possible semiconductor damage in the second subnetwork cannot produce a conductive connection to the second subnetwork and therefore cannot produce a life-threatening contact voltage, for example. The multi-level converter used to provide the electrical energy has a low weight and requires only a small installation space. A DC-isolating converter function can be implemented via the multi-level converter using at least one converter. In one configuration, the, for example, modular multi-level converter is in the form of an M2SPC (modular multi-level parallel/serial converter) and comprises capacitors and/or batteries as a plurality of energy stores or components of the individual modules of the multi-level converter.

The multi-level converter comprising a plurality of individual modules is used as a central energy store of the energy supply system, in which case the multi-level converter is to be used to produce a high voltage inside the first subnetwork. Starting from this first high voltage of the multi-level converter, the transformer is used to provide a comparatively low voltage for the second subnetwork, these two subnetworks additionally being DC-isolated via the transformer. The voltage provided by the multi-level converter is subject only to minor fluctuations. A plurality of batteries as energy stores can be dynamically reconfigured using the multi-level converter and can therefore also be used for a motor vehicle.

In one configuration, the usually modular multi-level converter is used to produce the AC voltage for the first subnetwork, which has the high value of the voltage, from a plurality of energy stores of the individual modules which are in the form of DC voltage sources, for example. Instead of a converter which can otherwise be used, the second subnetwork is connected to the first subnetwork via the transformer in the presented energy supply system, in which case it is possible to exchange energy between the two subnetworks. The transformer is supplied with electrical energy using the usually low second incoming AC voltage which is modulated onto the first incoming AC voltage. This second incoming AC voltage is modulated onto at least one phase, generally all phases, of the multi-level converter. A reference point which can be suitably selected in this case makes it possible to prevent the modulated second incoming AC voltage from influencing operation of the electrical machine.

Further advantages and configurations of the invention emerge from the description and the accompanying drawings.

It goes without saying that the features mentioned above and the features yet to be explained below can be used not only in the respectively stated combination but also in other combinations or alone without departing from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is schematically illustrated using embodiments in the drawings and is schematically described in detail with reference to the drawings.

FIGS. 1a, 1b and 1c show a schematic illustration of an electrical circuit known from the prior art.

FIGS. 2a, 2b, 2c and 2d show a schematic illustration of a first embodiment of the multi-level converter according to the invention and graphs for carrying out a first embodiment of the method according to the invention.

FIG. 3 shows a schematic illustration of a first embodiment of the energy supply system according to the invention.

FIG. 4 shows a schematic illustration of a second embodiment of the energy supply system according to the invention.

FIGS. 5a, 5b, 5c, 5d and 5e show a schematic illustration of the first embodiment of the multi-level converter according to the invention and graphs for carrying out a further embodiment of the method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The figures are described in an interrelated and comprehensive manner. The same reference numerals are assigned to identical components.

The circuit 2 schematically illustrated in FIG. 1a comprises an energy source 4 and a high-voltage load 6 which are both connected via an inverter 8 which is designed to convert a DC voltage produced by the energy source 4 into an AC voltage and to make it available to the high-voltage load 6.

In this case, as shown by the graph from FIG. 1b , it is possible to produce a variable average voltage by means of phase chopping control. Alternatively, as shown by the graph from FIG. 1c , a variable average voltage can be produced by means of three-point pulse width modulation.

The first embodiment of the multi-level converter 10 according to the invention, as schematically illustrated in FIG. 2a , comprises a first section 12 having four individual modules 14 a, 14 b, 14 c, 14 d, a second section 16 likewise having four individual modules 18 a, 18 b, 18 c, 18 d and a third section 20 having four individual modules 22 a, 22 b, 22 c, 22 d. In this case, it is possible to also refer to each of said sections 12, 16, 20 as an arm of the multi-level converter 10. This multi-level converter which is modular here is, for example, in the form of an MMC, an MMSPC or a Matroska converter which is described in the German patent application DE 102015112513.

Each of the individual modules 14 a, 14 b, 14 c, 14 d, 18 a, 18 b, 18 c, 18 d, 22 a, 22 b, 22 c, 22 d comprises at least one energy store, for example a capacitor or a battery, which is why the multi-level converter 10 has a plurality of distributed energy stores. Energy is to be made available here to a first phase of an electrical machine using energy stores of the individual modules 14 a, 14 b, 14 c, 14 d of the first section. Electrical energy is to be made available to a second phase of this electrical machine via the individual modules 18 a, 18 b, 18 c, 18 d of the second section 16. In addition, energy is to be made available to a third phase of the electrical machine using the individual modules 22 a, 22 b, 22 c, 22 d of the third section 20.

FIGS. 2b, 2c, 2d each comprise a graph having an abscissa 24, along which the time is plotted, and an ordinate 26, along which values of an electrical voltage are plotted. In this case, the first graph from FIG. 2b shows a profile 28 of a first incoming AC voltage which is required by a consumer or a load, here the electrical machine. In this case, provision is made for this machine to be directly connected to the multi-level converter 10 inside a first subnetwork which is not illustrated any further. A further consumer which requires an AC voltage having a lower value than the first consumer in the first subnetwork is arranged in a second subnetwork which is not illustrated any further and is to be connected to the first subnetwork via a DC-isolating transistor.

A profile 30 of the second incoming AC voltage of this second consumer is shown in the second graph from FIG. 2c , a comparison of the graphs from FIGS. 2b and 2c showing that the first incoming AC voltage for the first consumer has a higher amplitude than the second incoming AC voltage of the second consumer. In contrast, the frequency of the second incoming AC voltage of the second consumer has a higher frequency than that of the first consumer. When carrying out an embodiment of the method according to the invention, the second incoming AC voltage which is shown in the graph from FIG. 2c and has the profile 30 is added to the first incoming AC voltage having the profile 28 from FIG. 2b , a resulting voltage having a profile 32 as illustrated in FIG. 2d being produced, which resulting voltage is produced as a sum voltage during modulation of the first incoming AC voltage with the second incoming AC voltage and is used to excite the primary side of the transformer.

The first embodiment of the energy supply system 40, as schematically illustrated in FIG. 3, comprises a first subnetwork 42 and a second subnetwork 44. The first subnetwork 42 comprises a second embodiment of the multi-level converter 46 according to the invention which again has three sections 47, 49, 51 or arms which are connected in parallel with one another, a first such section 47 having a first individual module 48 a, a second individual module 48 b, a third individual module 48 c and a fourth individual module 48 d. A second section 49 of the multi-level converter 46 has a first individual module 50 a, a second individual module 50 b, a third individual module 50 c and a fourth individual module 50 d. In addition, the multi-level converter 46 comprises a third section 51 having a first individual module 52 a, a second individual module 52 b, a third individual module 52 c and a fourth individual module 52 d. In this case, all individual modules 48 a, 48 b, 48 c, 48 d, 50 a, 50 b, 50 c, 50 d, 52 a, 52 b, 52 c, 52 d each have an energy store in the form of a battery or a capacitor.

The multi-level converter 46 also comprises a monitoring unit 54 and a further energy store 56. One section 47, 49, 51 of the multi-level converter 42 is respectively assigned to one phase of a total of three phases U, V, W of an electrical consumer 58 which is in the form of an electrical machine here.

During operation of the multi-level converter 46, a value of an amplitude of a first incoming AC voltage, which is to be made available to a respective phase U, V, W of the consumer 58, is set via the monitoring unit 54. In this case, the first section 47 having the individual modules 48 a, 48 b, 48 c, 48 d is assigned to a first phase U. The second section 49 having the individual modules 50 a, 50 b, 50 c, 50 d is assigned to a second phase V of the consumer 58. In addition, the third section 51 having the individual modules 52 a, 52 b, 52 c, 52 d is assigned to the third phase W of the consumer 58.

All individual modules 48 a, 48 b, 48 c, 48 d, 50 a, 50 b, 50 c, 50 d, 52 a, 52 b, 52 c, 52 d have an identical design and each have an identical energy store which is to be used to respectively provide an AC voltage, the amplitude of which has the same value. Depending on the intended value of the amplitude of the AC voltage which is to be made available to a respective phase U, V, W, the monitoring unit 54 activates at least one individual module 48 a, 48 b, 48 c, 48 d, 50 a, 50 b, 50 c, 50 d, 52 a, 52 b, 52 c, 52 d, generally a plurality of individual modules 48 a, 48 b, 48 c, 48 d, 50 a, 50 b, 50 c, 50 d, 52 a, 52 b, 52 c, 52 d, inside a respective section 47, 49, 51, at least two individual modules 48 a, 48 b, 48 c, 48 d, 50 a, 50 b, 50 c, 50 d, 52 a, 52 b, 52 c, 52 d inside a respective section 47, 49, 51, for example, being connected in series and/or in parallel with one another depending on the value of the amplitude of the AC voltage to be provided.

In this case, provision is made for the first subnetwork 42 to be operated with a voltage which is higher than a second voltage of the second subnetwork 44. In this case, both subnetworks 42, 44 are connected to one another via a DC-isolating transformer 60, the primary side of the transformer 60 being assigned to the first subnetwork 42 and a secondary side of the transformer 60 being assigned to the second subnetwork 44. In addition, a rectifier 62, to which an energy store 64 is connected, is connected downstream of the transformer 60 inside the second subnetwork 44.

A reference point 66, which is at a minimum potential of the multi-level converter 46 here, is also defined for the multi-level converter 46. On the one hand, the primary side of the transformer 60 is connected to the reference point 66 here. In addition, the primary side is also connected to a connection point which connects the third section 51 of the multi-level converter 46 to the third phase W of the consumer 58.

When carrying out an embodiment of the method according to the invention, a three-phase system is provided with the individual modules 48 a, 48 b, 48 c, 48 d, 50 a, 50 b, 50 c, 50 d, 52 a, 52 b, 52 c, 52 d of the respective section 47, 49, 51.

In this case, the first section 47 is assigned to a first phase U, the second section 49 is assigned to a second phase V and the third section 51 is assigned to a third phase W of the consumer 58. In the method, a potential of the reference point 66 is not clearly stipulated, but rather is adjusted to a neutral point voltage of the three phases U, V, W as a virtual neutral point.

In one embodiment of the method, a second incoming AC voltage is modulated as a high-frequency harmonic onto a first incoming AC voltage between the third phase W and the reference point 66 and is used as an input for the transformer 60 for DC isolation, but at the same time is not visible to the consumer 58. This is achieved, for example, when voltages V_(w-r), V_(u-r) and V_(v-r) between a respective phase U, V, W and the reference point 66 are identical, the following applying to differences in voltages between two phases U, V, W in each case: V_(u-v)=V_(u-r)−V_(v-r), V_(u-w)=V_(u-r)−V_(w-r) and V_(u-w)=V_(u-r)=V_(w-r).

Integrated DC-isolated supply of consumers of the second subnetwork 44 is enabled via the transformer 60 using the multi-level converter 46 which is in the form of a three-phase MMSPC here.

The second embodiment of the energy supply system 70 according to the invention, as schematically illustrated in FIG. 4, comprises a first subnetwork 72 and a second subnetwork 74. The first subnetwork 42 comprises a third embodiment of the multi-level converter 76 according to the invention. In this case, provision is made for the third embodiment of the multi-level converter 46 to be largely structurally identical to the second embodiment of the multi-level converter 46. In addition, the two subnetworks 72, 74 of the second embodiment of the energy supply system 70 according to the invention comprise the same components as the first embodiment of the energy supply system 40 according to the invention from FIG. 3.

In this case too, the maximum amplitude with the first value is provided for a respective phase U, V, W of the consumer 58 by connecting the individual modules 48 a, 48 b, 48 c, 48 d, 50 a, 50 b, 50 c, 50 d, 52 a, 52 b, 52 c, 52 d of a respective section 47, 49, 51 in series and/or in parallel. A second or secondary incoming AC voltage, the amplitude of which has a lower second value, is also modulated onto this at least one first or primary incoming AC voltage. However, this second incoming AC voltage has a higher frequency than the first incoming AC voltage.

The second embodiment of the energy supply system 70 differs from the first embodiment from FIG. 3 in that the primary side of the transformer 60 between the two subnetworks 72, 74 is connected to the reference point 66 and to a connection between a third individual module 52 c and a fourth individual module 52 d of the third section 51 of the multi-level converter 46.

FIG. 5a likewise shows the first embodiment of the multi-level converter 10 according to the invention known from FIG. 2 a.

In this case, FIG. 5a also shows a horizontal arrow 13, along which a phase-phase voltage of the multi-level converter 10 is produced, which phase-phase voltage in this case corresponds to a voltage difference between the AC voltages between two sections 12, 16, 20 in each case. A value of a respective first or primary incoming AC voltage of a respective section 12, 16, 20 results along a vertically oriented arrow 15, which voltage is dependent on how many individual modules 14 a, 14 b, 14 c, 14 d, 18 a, 18 b, 18 c, 18 d, 22 a, 22 b, 22 c, 22 d of a respective section 12, 16, 20 contribute to providing the respective first section-specific or phase-specific incoming AC voltage on the basis of a series and/or parallel connection of the individual modules 14 a, 14 b, 14 c, 14 d, 18 a, 18 b, 18 c, 18 d, 22 a, 22 b, 22 c, 22 d.

FIGS. 5b, 5c, 5d each comprise a graph having an abscissa 24, along which the time is plotted, and an ordinate 26, along which values of an electrical voltage are plotted. In this case, the first graph from FIG. 5b comprises a profile 80 of a first primary incoming AC voltage or phase which is to be provided by connecting the individual modules 14 a, 14 b, 14 c, 14 d of the first section 12 of the multi-level converter, that is to say by connecting said individual modules in series and/or in parallel. A second profile 82 represents a second primary incoming AC voltage or phase of the multi-level converter 10 which is provided via its second section 16 containing the individual modules 18 a, 18 b, 18 c, 18 d. A third profile 84 shows a third primary incoming AC voltage or phase which is provided by connecting the individual modules 22 a, 22 b, 22 c, 22 d of the third section 20 of the multi-level converter 10. In this case, these three primary and therefore first incoming AC voltages have the same frequency and the same amplitude. In addition, these primary incoming AC voltages are phase-shifted through 120° relative to one another.

The graph from FIG. 5c shows a profile 86 of a second or secondary incoming AC voltage which is again modulated onto the three primary incoming AC voltages.

Profiles 88, 90, 92 of resulting voltages, here of resulting AC voltages, are illustrated in the graph from FIG. 5 d.

In this case too, provision is made for the multi-level converter 10 to be arranged in a first subnetwork which likewise has a three-phase consumer which is provided with the three primary incoming AC voltages. This subnetwork is also connected to a second subnetwork via a transformer, a value of the amplitude of the voltage inside the first subnetwork being greater than the value of the amplitude of the voltage in the second subnetwork.

FIG. 5e shows a schematic illustration of a controller 94 of a consumer of the first subnetwork, via the three phases U, V, W of which the multi-level converter 10 is to provide the three primary incoming AC voltages, the profiles 80, 82, 84 of which are illustrated in FIG. 5b . In this case, the first phase U is assigned to the first section 12, the second phase V is assigned to the second section 16 and the third phase W is assigned to the third section 20 of the multi-level converter 10. Furthermore, a monitoring unit 96 of the multi-level converter 10 produces the second or secondary incoming AC voltage, the profile 86 of which is shown in the graph from FIG. 5c . This second incoming AC voltage is added to the three first incoming AC voltages and is made available to the consumer, here an electrical machine.

Therefore, when regulating the electrical machine, the monitoring unit 94 provides the first incoming phase-specific AC voltages U, V, W or phase-specific currents. (Alternatively, it is possible for this monitoring unit 94 to be assigned to the multi-level converter 10 and to be designed to monitor operation of the electrical machine and to set the AC voltages and/or currents on the basis thereof.)

The monitoring unit 96 which is designed, inter alia, to monitor a DC-DC converter of the multi-level converter 10 produces the high-frequency second incoming AC voltage with a temporally variable amplitude and/or frequency which is provided, for example, for the purpose of regulating a power of consumers of the second subnetwork. Since the second incoming AC voltage is modulated onto all three first incoming AC voltages or phases, this is not visible to the consumer or the electrical machine of the first subnetwork. The second incoming AC voltage simultaneously corresponds to a difference between the reference point of the consumer and at least one reference point of the multi-level converter 10, the at least one such reference point usually being in the form of a neutral point. 

What is claimed is:
 1. A method for operating an electrical network comprising a first subnetwork and a second subnetwork which are connected to one another via a transformer and are DC-isolated from one another by the transformer, a primary side of the transformer with a first number of turns being assigned to the first subnetwork and a secondary side of the transformer with a second number of turns being assigned to the second subnetwork, the first subnetwork having a multi-level converter having a plurality of individual modules, each individual module having an electrical energy store, the multi-level converter providing at least one first incoming electrical AC voltage which is modulated with at least one second incoming electrical AC voltage, a resulting electrical voltage being made available to the transformer and being transformed by the transformer to an outgoing electrical voltage which is made available to the second subnetwork.
 2. The method as claimed in claim 1, in which provision is made for the at least one first incoming AC voltage to have an amplitude with a first value and a frequency with a first value, and for the at least one second incoming AC voltage to have an amplitude with a second value and a frequency with a second value, the first value of the amplitude of the at least one first incoming AC voltage being set to be greater than the second value of the amplitude of the second incoming AC voltage, and the first value of the frequency of the at least one first incoming AC voltage being set to be less than the second value of the frequency of the at least one second incoming AC voltage.
 3. The method as claimed in claim 1, in which the at least one first incoming AC voltage is modulated with the at least one second incoming AC voltage at a reference point of the multi-level converter.
 4. The method as claimed in claim 3, in which a neutral point of the multi-level converter is selected as the reference point.
 5. The method as claimed in claim 1, in which the at least one second incoming AC voltage is modulated onto the at least one first incoming AC voltage, the resulting voltage being provided as a sum of the incoming AC voltages.
 6. The method as claimed in claim 1, in which the multi-level converter provides a plurality of incoming first AC voltages which are phase-shifted with respect to one another and are modulated with the at least one second incoming AC voltage.
 7. A multi-level converter configured to be arranged in an electrical network, the electrical network comprising a first subnetwork and a second subnetwork, the first and second subnetworks are configured to be connected to one another via a transformer and are configured to be DC-isolated from one another by the transformer, a primary side of the transformer with a first number of turns is to be assigned to the first subnetwork and a secondary side of the transformer with a second number of turns is to be assigned to the second subnetwork, the multi-level converter is configured to be arranged in the first subnetwork and has a plurality of individual modules, each individual module having an electrical energy store, the multi-level converter being configured to provide at least one first incoming electrical AC voltage and to modulate the at least one first incoming electrical AC voltage with at least one second incoming electrical AC voltage, a resulting electrical voltage is to be (i) made available to the transformer, (ii) transformed by the transformer to an outgoing electrical voltage, and (iii) made available to the second subnetwork.
 8. The multi-level converter as claimed in claim 7, which is assigned a monitoring unit which is configured to set values of at least one physical parameter of either the first incoming electrical AC voltage or the second incoming electrical AC voltage.
 9. The multi-level converter as claimed in claim 7, in which at least two individual modules have the same design.
 10. The multi-level converter as claimed in claim 7, which is configured to produce the at least one first incoming AC voltage from an individual voltage from an energy store of at least one of the individual modules.
 11. The multi-level converter as claimed in claim 10, which is configured to connect at least two of the individual modules in series or in parallel with one another and to provide the at least one first incoming AC voltage from a combination of individual voltages of the at least two individual modules to be combined with one another.
 12. The multi-level converter as claimed in claim 7, which has a plurality of sections, each section having a combination of a plurality of individual modules connected to one another, and each section is respectively to be used to produce the first incoming AC voltage.
 13. The multi-level converter as claimed in claim 7, which is assigned at least one additional energy store which is configured to provide the at least one second incoming AC voltage.
 14. The multi-level converter as claimed in claim 7, in which energy stores of the individual modules are in the form of DC voltage sources, the multi-level converter having at least one converter which is configured to convert an individual voltage in the form of a DC voltage from an energy store of at least one of the individual modules into an AC voltage and to provide the at least one first incoming AC voltage therefrom.
 15. An energy supply system having an electrical network comprising a first subnetwork and a second subnetwork which are connected to one another via a transformer and are DC-isolated from one another by the transformer, a primary side of the transformer with a first number of turns being assigned to the first subnetwork and a secondary side of the transformer with a second number of turns being assigned to the second subnetwork, the first subnetwork having a multi-level converter having a plurality of individual modules, each individual module having an electrical energy store, the multi-level converter being configured to provide at least one first incoming electrical AC voltage and to modulate the first incoming electrical AC voltage with at least one second incoming electrical AC voltage, and a resulting electrical voltage is to be made available to the transformer, the transformer being configured to transform the resulting electrical voltage into an outgoing electrical voltage and to make the outgoing electrical voltage available to the second subnetwork.
 16. The energy supply system as claimed in claim 15, in which the first number of turns of the primary side of the transformer is greater than the second number of turns of the secondary side of the transformer.
 17. The energy supply system as claimed in claim 15, which is to be arranged in a motor vehicle.
 18. The energy supply system as claimed in claim 15, in which an electrical machine is assigned to the first subnetwork as consumer and has a plurality of phases, the multi-level converter being configured to respectively provide each phase with the first incoming electrical AC voltage. 